Oxygen activation can be used very effectively for determining the source of unwanted water production in producing oil and gas wells. Since the oxygen activation measurement is strictly tied to the presence of water, its ability to detect primarily water movement behind the casing is invaluable. Production Logging ("PL") equipment cannot resolve this kind of problem because PL sensors measure flow rate and density within the wellbore after hydrocarbon production from the zone of interest has commingled with water production, which may have come into the wellbore through a channel in the cement, either above or below the zone of interest.
An oxygen atom, when excited by high-energy neutrons, will emit gamma ray emissions to allow the atom to return to a ground state. The characteristic emission levels from this reaction are at 6.13 MeV 68% of the time and 7.12 MeV 4.9% of the time. The other 26% of the time, the atom returns directly to the ground state without an emission. The gamma ray population, after excitation has ceased, will decay exponentially, losing one-half of the total counts every 7.13 seconds. This 7.13-second period of time is called the half-life of oxygen activation. The percentage of counts existing at certain periods of time after neutron bombardment has ceased can be estimated from Table 1.
TABLE 1 ______________________________________ GAMMA RAY POPULATION % OF TOTAL COUNTS SECONDS AFTER BURST ______________________________________ 50. 7.13 25. 14.26 12.5 21.39 6.25 28.52 3.125 35.65 1.5625 42.78 ______________________________________
In addition to the decrease in counts available for counting due to the half-life decay of the emissions, the gamma rays that are emitted also undergo a reduction in energy level as they collide with other elements that lie between the point of emission and the detector of the tool. A large percentage of these counts will have energy levels substantially below the 6.13 or 7.13 MeV levels when the counts were emitted.
To enable the tool to be used effectively for evaluating producing wells, it must be able to monitor both upgoing and downgoing water channels. This can be accomplished only if the neutron source is positioned above the detectors as well as being able to log in the traditional neutron-source-below position. The tool can be inverted and sent downhole so that it can measure both upgoing and downgoing channels.
Prior art oxygen activation tools disclose the use of two detectors. Typical of such devices are shown in U.S. Pat. No. 4,737,636. This patent discloses a tool provided with two gamma detectors, D.sub.1, D.sub.2, and an ancillary gamma detector to measure background levels of radiation in the borehole. U.S. Pat. No. 3,603,795 discloses an oxygen activation downhole tool which includes a neutron source and two gamma ray detectors. U.S. Pat. No. 4,233,508 discloses a similar device for a water-injection profiling system with two detectors. U.S. Pat. Nos. 4,223,218; 3,603,795; and 3,979,300 also disclose a two-detector device. U.S. Pat. No. 4,743,755 also discloses a two-detector device, as does a paper entitled "Measuring Behind Casing Waterflow," by T. M. Williams of Texaco, Inc., given at the International Symposium on Subsurface Injection of Oilfield Brines in New Orleans, on May 5-7, 1987.
Also of interest in general background-type information regarding measurement of water flow behind and in wellbore casing is an article by D. M. Arnold and H. J. Paap entitled "Quantitative Monitoring of Water Flow Behind and In Wellbore Casing," published in the Jan. 1979 issue of the Journal of Petroleum Technology.
Also of interest regarding the field of invention is a paper published by W. H. M. DeRosset entitled "Examples of Detection of Water Flow by Oxygen Activation on Pulsed Neutron Logs." The paper emphasizes detection techniques, such as controlling the relative velocity of the tool to the water velocity in an effort to maximize the gamma response from activation. The technique deals with logging the well under static and flow conditions or logging a different tool speeds in a flowing well. The disclosure indicates a two-detector tool.
An article by P. A. Wichmann, entitled "Advances in Nuclear Production Logging," published in the SPWLA Transactions (1967), deals with use of gate which allows a gamma ray discriminator to be set at a lower value, which in turn substantially increases the count rate as shown in FIG. 4. Three gates are employed only for this purpose.
U.S. Pat. No. 4,327,290 relates to a method and apparatus for the cyclical timing of the neutron bursts, spectral gates, tau determination gates, and background measurement periods in a capture spectroscopy measurement. U.S. Pat. No. 3,887,805 discusses a neutron logging tool involving two neutron detectors, coupled with a third neutron detector, or its physical equivalent, positioned within the tool at a distance relative to the neutron source and the other detector, which enables changes in the signal from the third detector caused by variations in the borehole environment to be generally distinguishable from changes in signal from at least one of the other detectors. The signals from the third detector and one of the two other detectors are combined in order to compensate for the influence of borehole environment. The patent teaches the two detectors spaced approximately 13 and 15 inches, respectively, from the neutron source, with the third detector being closer to the neutron source and located in such a manner that the response to borehole variations as to the closest detector to the neutron source is different from the changes in signal from the other detector. The tool obtains readings from only two of the three detectors in an effort to compensate for the influence of the borehole environment.
In a paper by G. Lamb and G. Webber, presented at the SPWLA 24th Annual Logging Symposium in Jun. of 1983, entitled "Measurement of Water Flow in Deviated Production Wells by Oxygen Activation Logging," a tool is discussed having a neutron source and two gamma detectors at 40 and 61 centimeters, respectively, from the source. A third detector is used for natural gamma measurements and is spaced at 5.76 meters from the source. The paper indicates that the natural gamma detector can be usable for oxygen activation flow measurements. The paper indicates that the two near detectors are unusable due to direct activation of water in their vicinity. In order to make comparisons of the counts of the natural gamma and the far detectors, their different responses can be accounted for by scaling the response of the natural gamma detector to match that in the far detector, using the ratio of their responses in a radioactive shell.
The process of determining the presence and origin of water channeling behind the casing in a producing well is ideally suited to oxygen activation. The technique can determine where water flow exists behind casing while the well is under production. A properly designed tool should be able to detect water flow of various rates without contaminating production, damaging clays, or cutting off production.
The Environmental Protection Agency ("EPA") has a system whereby it classifies all injection wells in the United States. The main classification of immediate interest is the Class II well used by oil producers either for enhanced oil recovery ("EOR") or salt water disposal. The EPA, through various laws enacted by the U.S. Congress over the last several years, requires that operators of Class II wells verify that the injection operations are not contaminating underground sources of drinking water ("USDW"). The EPA regulations specifically require that a well have no "significant leak," but have not specified what volume of leak is "significant" to date. In other words, if a leak can be detected, then it is "significant" and must be eliminated. The EPA requires a Mechanical Integrity ("MI") test on each well every five years to prove that no leak exists from either the inject zone or an intervening zone to an USDW. An intervening zone is any zone between the inject zone and the USDW. Additionally, if there is another well of any type within the area of review of an injection well, the EPA requires the operator of that well to also prove that no fluids can migrate through that well to USDW's.
The need to measure down to very low (a few ft/min.) and very high fluid velocities has prompted those in the field to produce tools to accomplish this purpose. Recently proposed is a tool which had previously been used exclusively for formation testing. This tool has a neutron source and two gamma ray detectors. The tool is also outfitted with gates which regulate the cycling of the neutron source and the measurement time at the various detectors. This tool was set up for a sequence of approximately twenty to thirty short cycles of measurement, followed by one long cycle. The tool is calibrated by placing it in a zone which is assumed to be free of water flow, but otherwise showing characteristics identical to those of the region of the well under investigation. These short measuring periods are of insufficient length to get accurate background measurements. Accordingly, there is a statistical inaccuracy in the measurements, especially in a situation involving low count rates. Statistically, this tool is able to take meaningful measurements of the background level of radiation only approximately 17% of the time.
The apparatus and method of the present invention seek to optimize background measurements to reduce statistical error and make the oxygen activation measurements more accurate. In accomplishing this optimization objective, the additional detectors are used. The use of the additional detectors improves the rangeability of the tool to approximately a range of water flow rates of about 11/2 ft/min. to about 50 ft/min. In furthering such optimization of measurements, the tool is equipped to take meaningful background measurements more frequently than the above-mentioned known tool, thereby making the oxygen activation measurements more meaningful.